1. Field of the Invention
The present invention relates to a labeled complex for microassay using near-infrared radiation. More specifically, the present invention relates to a labeled complex capable of specifically detecting a certain particular component in a complex mixture with a higher sensitivity.
2. Related Background Art
On irradiating a laser beam on a trace substance labeled with dyes and the like, information due to the substance is generated such as scattered light, absorption light, fluorescent light and furthermore light acoustics. It is widely known in the field of analysis using lasers, to detect such information so as to practice microassays rapidly with a higher precision.
A gas laser represented by an argon laser and a helium laser has conventionally been used exclusively as a laser source. In recent years, however, a semiconductor laser has been developed, and based on the characteristic features thereof such as inexpensive cost, small scale and easy output control, it is now desired to use the semiconductor laser as a light source.
If diagnostically useful substances from living organisms are assayed by means of the wavelength in ultraviolet and visible regions as has conventionally been used, the background (blank) via the intrinsic fluorescence of naturally occurring products, such as flavin, pyridine coenzyme and serum proteins, which are generally contained in samples, is likely to increase. Only if a light source in a near-infrared region can be used, such background from naturally occurring products can be eliminated so that the sensitivity to substances to be measured might be enhanced, consequently.
However, the oscillation wavelength of a semiconductor laser is generally in red and near-infrared regions (670 to 830 nm), where not too many dyes generate fluorescence via absorption or excitation. A representative example of such dyes is polymethine-type dye having a longer conjugated chain. Examples of labeling substances from living organisms with a polymethine-type dye and using the labeled substances for microanalysis are reported by K. Sauda, T. Imasaka, et al. in the report in Anal. Chem., 58, 2649-2653 (1986), such that plasma protein is labeled with a cyanine dye having a sulfonate group (for example, Indocyanine Green) for the analysis by high-performance liquid chromatography.
Japanese Patent Application Laid-open No. 2-191674 discloses that various cyanine dyes having sulfonic acid groups or sulfonate groups are used for labeling substances from living organisms and for detecting the fluorescence.
However, these known cyanine dyes emitting fluorescence via absorption or excitation in the near-infrared region are generally not particularly stable under light or heat.
If the dyes are used as labeling agents and bonded to substances from living organisms such as antibodies for preparing complexes, the complexes are likely to be oxidized easily with by environmental factors such as light, heat, moisture, atmospheric oxygen and the like or to be subjected to modification such as generation cross-links. Particularly in water, a modification such as hydrolysis is further accelerated, disadvantageously. Therefore, the practical use of these complexes as detecting reagents in carrying out the microassay of the components of living organisms has encountered difficulties because of their poor stability under storage.